How exactly are you writing to the other image? If it's not through the blend unit, it might be incrementing whenever the fragment program is run, not whenever it actually produced a pixel. If so, that would explain the doubling up, as on most chips fragment programs get run in 2x2 blocks -- thus running the fragment program twice on every pixel along the diagonal.
The clipping shouldn't be generating new triangles as long as your fullscreen tri stays within a reasonable range. IIRC, for +/- 1 NDC space, the vertices you need are (-1, -1) (3, -1) (-1, 3). http://www.gamedev.net/topic/568915-...reen-triangle/ has a bit more detail on it.